Overview
Note that in this disclosure the term "ultrasonic cleaning" will be interpreted to mean ultrasonic cleaning and/or degreasing, as the systems and methods by which a cleaning target is degreased are generally considered to be a specific application of ultrasonic cleaning systems and methods. Similarly, the term "method" will be considered to encompass the various processes by which ultrasonic cleaning and/or degreasing may be affected.
Ultrasonic cleaning has found its most successful application in the removal of insoluble particulate contamination from hard substrate surfaces of what will generally be described as a cleaning target, or item to be cleaned. Contamination that is insoluble or emulsified can usually be removed with facility by means of conventional methods in conjunction with suitable solvents or detergent solutions.
Such techniques, however, cannot adequately remove particulate matter in the micron and sub-micron size range to the extent that is necessary, for example, for the critical cleaning required in the microelectronics and optical industries or for the preparation of surfaces prior to the application of thin films or coatings.
A number of methods have been used for the purpose of removing microparticulates from hard surfaces. These include pressure spraying or manual and mechanical scrubbing with solvents or detergent solutions; vapor degreasing; ion bombardment; plasma, chemical, or ultrasonic cleaning; and ultraviolet/ozone cleaning.
Ultrasonic Cleaning Principles
Ultrasonic cleaning consists of immersing a part to be cleaned (cleaning target) in a suitable liquid medium (cleaning fluid), agitating or sonicating that medium with a high frequency (typically 18 to 120 kHz) sound waves for a brief period of time (usually a few minutes), rinsing with clean solvent or water, and drying. The mechanism underlying this process is one in which microscopic bubbles in the liquid medium implode or collapse under the pressure of agitation to produce shock waves which impinge on the surface of the part. Through a scrubbing action, these shock waves displace or loosen particulate matter from the surface of the cleaning target. The process by which these bubbles collapse or implode is known as cavitation.
High intensity ultrasonic sound waves are known to exert powerful forces that are capable of eroding even the hardest surfaces. Quartz, silicon, alumina, and other materials can be etched by prolonged exposure to ultrasonic cavitation, and cavitation burn has been encountered following repeated cleaning of glass surfaces. The severity of this erosive effect has been known to preclude the use of ultrasonics in the cleaning of some sensitive or delicate components.
Sound Wave Types
A sound wave is produced when a solitary or repeated displacement is generated in a sound conducting medium, such as by a "shock" event or "vibratory" movement. The displacement of air by the cone of a radio speaker is a good example of "vibratory" sound waves generated by mechanical movement. As the speaker moves back and forth, the air in front of the cone is alternately compressed and rarefied to produce sound waves, which travel through the air until they are finally dissipated. These sound waves are produced by generating an alternating mechanical motion. There are also sound waves which are created by a single "shock" event. Thunder is an example, and in this case the air experiences an instantaneous change in volume as a result of an electrical lightning discharge. Shock events are sources of a single compression wave which radiates from the source.
Compression and Rarefaction
As a sound wave travels through a sound conducting medium, the molecules in the medium are influenced by adjacent molecules in much the same way the coils of a spring influence one another when they are alternately compressed or expanded.
Cavitation and Implosion
The compression and rarefaction described above may be described in terms of the coils of a spring similar to a Slinky toy spring. Here the coils of the Slinky toy spring represent individual molecules of a sound conducting medium. The molecules in the medium are influenced by adjacent molecules in much the same way that the coils of the spring influence one another. The compression generated by the sound source as it moves propagates down the length of the spring as each adjacent coil of the spring pushes against its neighbor. It is important to note that, although the wave travels from one end of the spring to the other, the individual coils remain in their same relative positions, being displaced first one way and then the other as the sound wave passes. As a result, each coil is first part of a compression as it is pushed toward the next coil and then part of a rarefaction as it recedes from the adjacent coil. In much the same way, any point in a sound conducting medium is alternately subjected to compression and then rarefaction. At a point in the area of a compression, the pressure in the medium is positive. At a point in the area of a rarefaction, the pressure in the medium is negative.
Cavitation
In elastic media such as air and most solids, there is a continuous transition as a sound wave is transmitted. In non-elastic media such as water and most liquids, there is continuous transition as long as the amplitude or `loudness` of the sound is relatively low. As amplitude is increased, however, the magnitude of the negative pressure in the areas of rarefaction eventually becomes sufficient to cause the liquid to fracture because of the negative pressure, causing a phenomenon known as cavitation. Cavitation `bubbles` are created at sites of rarefaction as the liquid fractures or tears because of the negative pressure of the sound wave in the liquid. As the wave fronts pass, the cavitation `bubbles` oscillate under the influence of positive pressure, eventually growing to an unstable size. Finally, the violent collapse of the cavitation `bubbles` results in implosions, which cause shock waves to be radiated from the sites of the collapse. The collapse and implosion of myriad cavitation `bubbles` throughout an ultrasonically activated liquid result in the effect commonly associated with ultrasonics. It has been calculated that temperatures in excess of 10,000.degree. F. and pressures in excess of 10,000 PSI are generated at the implosion sites of cavitation bubbles. It is these temperatures and pressures which account for the cleaning action observed in an ultrasonic tank.
Ultrasonic Harmonics
A given cleaning target may be contaminated with a wide variety of particulate matter having a wide range of particle sizes. It has been shown that to affect cleaning of cleaning targets with smaller particulate matter requires the cavitation of smaller and smaller air bubbles within the ultrasonic cleaning fluid. In the past this has been accomplished by using square wave excitation to "shock" the ultrasonic cleaning fluid with both a fundamental ultrasonic frequency and harmonics of this frequency. The higher frequency harmonics then resonate the smaller air bubbles and thus affect greater cavitation at the cleaning target surface.
As cleaning requirements have become more stringent, this approach has been found to be wanting since only the air bubbles of the size susceptible to resonance at the harmonics of the fundamental ultrasonic frequency are impacted by the use of a square wave excitation. Recent advances in ultrasonics have moved towards swept frequency ultrasonic excitations in which a wide range of fundamental frequencies is swept to thus generate a corresponding wide range of high frequency harmonics. The primary purpose for these advances has been to obtain more effective and more rapid cleaning of the cleaning target. Unfortunately, these techniques generally drastically increase the cost of the ultrasonic cleaning system.
Alternatives to this approach have attempted to generate ultrasonic energy sources which have higher amplitude and wider spectrum harmonic resonances in order to affect cavitation of smaller and smaller air bubbles. These techniques, while moderately successful, are generally expensive. As with the swept frequency technique, ultrasonic manufacturers are concentrating on making the ultrasonic energy source more efficient rather than improving the efficiency of the entire ultrasonic cleaning system.
Summary
Thus, from the foregoing discussion, it can be surmised that ultrasonic cleaning manufacturers have gone to great lengths to use ultrasonic harmonics to increase cavitation and thus affect faster and more effective cleaning. The ever-increasing requirements for higher quality cleaning, such as in the semiconductor and related industries, requires that cavitation of smaller air bubbles be performed, and this in general requires higher frequencies to be used to excite the ultrasonic cleaning bath. Furthermore, since there are in general many small bubbles to be cavitated, the amount of high frequency ultrasonic energy which is required to affect cavitation must be increased accordingly. There is a practical limit to increasing this energy level using conventional square-wave ultrasonic generators, and the use of other forms of ultrasonic excitation, such as piezoelectric methods, can be expensive.